Control apparatus for vibration type actuator

ABSTRACT

The present invention relates to a control apparatus for a vibration type actuator, and particularly to a control apparatus which improves controllability by providing a control circuit of adjusting a voltage to be applied to the actuator so that a tilt of a frequency-speed characteristic of the actuator becomes approximately a constant tilt.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a control apparatus for avibration type actuator such as a vibration wave motor or the like.

[0003] 2. Related Background Art

[0004] Generally, a vibration type actuator such as a vibration wavemotor or the like includes a vibration member for making drivingvibration and a contact member for coming into contact with thevibration member in pressurization, and causes the vibration member andthe contact member to relatively move by the driving vibration.

[0005] Then, the vibration member generally consists of an elasticmember and a piezoelectric element functioning as an electromechanicalenergy conversion element. For example, the piezoelectric element isdisposed so as to have the driving phase at the position havingspatially a mutual phase difference of 90° for the driving phase of theelastic member, alternating signals of two phases having a mutual phasedifference of 90° are applied to these two driving phases to generate atravelling wave on the elastic member, and the contact member ispressure-contacted with the elastic member, thereby obtaining drivingforce frictionally.

[0006] Here, it should be noted that a frictional material for obtainingthe appropriate frictional force is adhered, coated or formed at thecontact portion between the vibration member and the contact member.

[0007] With respect to the features of the vibration type actuator, ascompared with an actuator using electromagnetic force, several pointsthat driving torque at low speed is large, responsiveness is excellent,and it is silent because, as the vibration over an audible range isused, humans can not feel any driving sound are enumerated. Therefore,the vibration type actuator is suitably used as, e.g., the driving unitof an image formation apparatus.

[0008] Generally, since a large voltage is necessary for the vibrationtype actuator, the voltage is boosted or risen by one method or acombination of plural methods. For example, a driving signal isamplified by a linear amplifier, the voltage is boosted by atransformer, or an inductance element and a switching element arecombined and thus a resonance with the capacitance component of thevibration type actuator is used.

[0009] In these methods described above, the method of boosting thevoltage by the transformer or the method of boosting the voltage bycombining the inductance element and the switching element is desirablyused because it is excellent in respects of efficiency, costs and thelike.

[0010] Moreover, as methods of controlling the driving speed of thevibration type actuator, there are a method of controlling the drivingspeed by using a driving voltage, a method of controlling the drivingspeed by using a driving frequency and a method of controlling thedriving speed by using a phase between adjacent driving phases. In thesemethods, the method of controlling the driving speed by using thedriving frequency is desirably used because it can achieve both a widedynamic range and high resolution singly and is excellent in conformitywith a recently developed digital circuit.

[0011] However, in the driving speed control method using the drivingfrequency, as shown in FIG. 4, a frequency-speed characteristic changesgreatly according to a frequency, whereby there is a problem that achange rate of the speed varies even at the same control operationamount.

[0012] Particularly, if the frequency is apart from a resonancefrequency (fr), a tilt (i.e., the tilt of the frequency for the speed)decreases, whereby there is a problem that a necessary control gain cannot be obtained and the speed does not decrease.

[0013] That is, there is a problem that controllability deteriorates ina low-speed range. Besides, if the control gain is set at low speed,there is a problem that oscillation occurs in high-speed driving.Particularly, when the vibration type actuator is used in positioningcontrol, there is a problem that a desired device can not be accuratelystopped at a desired position.

SUMMARY OF THE INVENTION

[0014] An object of the invention is to provide a control apparatus, fora vibration type actuator, which achieves steady driving by a simplemanner in a wide range of the driving from high speed to low speed.

[0015] One aspect of the invention is to provide a control apparatus fora vibration type actuator, which makes driving vibration at a drivingunit of a vibration member by applying an alternating signal to anelectromechanical energy conversion element and uses at least afrequency of the alternating signal as a speed control parameter, theapparatus comprising:

[0016] a driving circuit capable of changing a driving voltage of thealternating signal to be applied to the electromechanical energyconversion element; and a control circuit for controlling the drivingcircuit so that at least an absolute value of a tilt of afrequency-speed characteristic of the actuator is within a predeterminedrange in a frequency band of predetermined range.

[0017] One aspect of the invention is to provide a control apparatus fora vibration type actuator, which makes driving vibration at a drivingunit of a vibration member by applying an alternating signal to anelectro-mechanical energy conversion element and uses at least afrequency of the alternating signal as a speed control parameter, theapparatus comprising:

[0018] a driving circuit capable of changing a driving voltage of thealternating signal to be applied to the electro-mechanical energyconversion element; and a control circuit for controlling the drivingcircuit so that an absolute value of a tilt of a frequency-speedcharacteristic of the actuator is a predetermined value or more at leastin a frequency band of predetermined range.

[0019] One aspect of the invention is to provide a control apparatus fora vibration type actuator, which makes driving vibration at a drivingunit of a vibration member by applying an alternating signal to anelectromechanical energy conversion element and controls at least afrequency of an alternating signal as a speed control parameter, theapparatus comprising:

[0020] a driving circuit capable of changing a driving voltage of thealternating signal to be applied to the electromechanical energyconversion element; and a control circuit for at least performingcontrol in a frequency range higher than a predetermined frequency sothat the driving voltage to be applied to the electro-mechanical energyconversion element by the driving circuit decreases as the predeterminedfrequency becomes a higher frequency.

[0021] Other objects of the invention will become apparent from thefollowing embodiments which will be explained with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a block diagram showing the first embodiment of theinvention;

[0023]FIG. 2 is a cross-sectional view showing the structure of anexample of a vibration type actuator in the invention;

[0024]FIG. 3 is a block diagram showing a conventional control apparatuscorresponding to the invention;

[0025]FIG. 4 is a view showing a frequency-speed characteristic of thevibration type actuator in the invention;

[0026]FIG. 5 is a view showing an example of a speed command in positioncontrol;

[0027]FIG. 6 is a view showing the frequency-speed characteristics ofthe vibration type actuator, in the first embodiment of the inventionand a conventional control circuit;

[0028]FIG. 7 is a view showing the frequency-speed characteristics ofthe vibration type actuator, in the modification of the first embodimentand the conventional control circuit;

[0029]FIG. 8 is a block diagram showing the second embodiment of theinvention;

[0030]FIG. 9 is a block diagram showing the third embodiment of theinvention;

[0031]FIG. 10 is a block diagram showing the fourth embodiment of theinvention;

[0032]FIG. 11 is a view showing pulses for driving a MOSFET in aconventional example;

[0033]FIG. 12 is a view showing a state that pulses for driving a MOSFETare squeezed in the invention;

[0034]FIG. 13 is a view showing the frequency-speed characteristic ofthe vibration type actuator in the first embodiment of the invention;

[0035]FIG. 14 is a block diagram showing the fifth embodiment of theinvention; and

[0036]FIG. 15 is a view showing an operation in the fifth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] (First Embodiment)

[0038]FIG. 1 is a block diagram showing the first embodiment of theinvention, FIG. 2 is a cross-sectional view showing an example of avibration type actuator which can effectively perform the invention, andFIG. 3 is a block diagram showing a conventional positioning circuitcorresponding to the circuit shown in FIG. 1.

[0039] In the vibration type actuator shown in FIG. 2, both sides of alaminated piezoelectric element 17 which has plural sets of two drivingphases disposed at positions having spatially a mutual phase differenceof 90° are put and supported between elastic members 16 and 18. Byapplying alternating signals of two phases having a mutual phasedifference of 90° to the two driving phases of the piezoelectric element17, travelling waves as driving vibrations are generated on the outersurfaces of the respective elastic members 16 and 18, and rotationmembers 15 and 19 functioning as contact members are pressure-contactedwith the elastic members 16 and 18 respectively, thereby obtainingdriving force based on frictional force. Here, for example, pluralranges respectively having different polarization directions areprovided in each driving phase. Thus, for one driving phase,displacements of expansion and shrinking are simultaneously given in thethickness direction (axial direction) by applying the sine-wavealternating signals to the ranges of the different polarizationdirections, whereby bending vibration is made. Similarly, for the otherdriving phase, the bending vibration is made by applying the cosine-wavealternating signals. Moreover, in a case where the polarization rangesof the respective driving phases are turned to the same polarizationdirection, the phase-inverted alternating signals are applied.

[0040] When the actuator is driven, pulses having arbitrary pulse widthsand frequencies and having a mutual phase difference of 180° are appliedto the gates of MOSFET's (metal oxide semiconductor field-effecttransistors) 7 and 8 for the one driving phase connected to a coil 11and to the gates of MOSFET's 9 and 10 for the other driving phaseconnected to a coil 12.

[0041] That is, the switching pulse is set to have the phases 0°, 180°,90° and 270° in due order from g1 to g4 with the pulse width ofapproximately 50%, as shown in FIG. 11. When the pulse is inverted, itis set to have the phases 0°, 180°, −90° and −270° in due order from g1to g4. The value of the coil is set to match the capacitance of thevibration type actuator. Actually, the resonance frequencies of the coiland the capacitance are set to be higher than the resonance frequency ofthe actuator to moderate a change rate of a voltage.

[0042]FIG. 3 shows an example of the conventional positioning controlcircuit. In the control circuit of a conventional vibration typeactuator 13, a speed signal v of a speed detection means 14 such as aknown rotary encoder or the like for detecting the rotation of thevibration type actuator 13 is converted into a position signal P by aposition counter 5. Then, a speed command Vc according to the currentposition is generated by a position control block 2 to reach the targetposition, e.g., as shown in FIG. 5. Further, a frequency f of the pulseto drive the vibration type actuator is determined based on the speedcommand Vc, the speed signal V from the speed detection means 14, acontrol gain and the like by a speed control block 3, and the determinedfrequency f is output to a pulse generator 6.

[0043] A pulse width PW of a pulse generated by the pulse generator 6 isset to have a predetermined value irrespective of the command frequencyf.

[0044] The pulses of four phases are generated based on the commandfrequency f and the pulse width PW by the pulse generator 6 to driveMOSFET's 7 to 10, whereby the vibration type actuator 13 is driventhrough coils 11 and 12.

[0045] Since the vibration type actuator 13 has the frequency-speedcharacteristic (i.e., the speed characteristic for the change in a unitamount of the frequency) as shown in FIG. 4, the speed can be controlledby adjusting the frequency. However, since the tilt of thefrequency-speed characteristic changes greatly according to thefrequency, there is a fear that satisfactory control can not beperformed according to the speed range. Particularly, the gain does notsuffice in low speed. In the positioning control, to improve the stopaccuracy and decrease an impulsive sound at the time of start and stop,the speed control as shown in FIG. 5 is performed. In this case, it isnecessary to perform the steady speed control within a wide speed range.Particularly, the stability in the low-speed range is important.Moreover, since the speed does not decrease enough in a predeterminedfrequency range, there is a fear that it causes an overrun.

[0046] On the other hand, in the first embodiment of the invention, asshown in FIG. 1, a pulse width PW corresponding to a frequency command fgenerated by a speed control block 3 is stored beforehand in a knownmemory device 4 such as a RAM, a ROM or a gate array, and thus the pulsewidth PW according to the frequency command f is output to a pulsegenerator 6.

[0047] A table for the frequency commands f and the pulse widths PW isset beforehand by experiment or study so that the absolute value of thetilt of the frequency-speed characteristic can secure a gain enough forthe control or can be set in a predetermined range. FIG. 6 shows thefrequency-speed characteristics in the conventional art and the firstembodiment.

[0048] In the first embodiment, the pulse width for the frequency isdetermined so that the frequency-speed characteristic almost becomes astraight line. That is, the pulse width is maximum at a point a(frequency fa), and the pulse width is squeezed at the above and belowof the point a (above and below of frequency fa). FIG. 12 shows thestate that the pulses are squeezed with the frequency same as that inFIG. 11. As a result, since the response to the control command becomesthe same at any frequency, the steady control can be performed in thewide speed range from high speed to low speed, whereby it is suitablefor the positioning control.

[0049] Incidentally, it is difficult to make the frequency-speedcharacteristic linear accurately and completely. However, as shown inFIG. 13, there is no problem even if a range of the tilt of thefrequency-speed characteristic is determined and the tilt is made tobe-put within this range.

[0050] Besides, if the necessary gain only has to be secured, it is alsoeffective to set the absolute value of the tilt of the frequency-speedcharacteristic of the vibration type actuator to be a predeterminedvalue or more as shown in FIG. 7. In this case, the pulse width issqueezed at the frequency above a point a (frequency fa). Although thefrequency-speed characteristic does not become linear entirely in theused frequency range, the control can be performed in the wide speedrange because the necessary control gain can be secured. In this case,the control becomes possible even as for higher speed farther. Althoughthe apparatus for positioning control was explained in the firstembodiment, the same effect as above can be obtained if the embodimentis applied to an apparatus only for speed control.

[0051] (Second Embodiment)

[0052]FIG. 8 shows the second embodiment of the invention.

[0053] It should be noted that, in the second embodiment, theexplanation of the same parts as those in the first embodiment will beomitted. According to the second embodiment, in a speed control block 3which consists of a logic circuit such as a known CPU, a gate array orthe like, a reduction number ΔPW(f) of the pulse width is calculatedfrom a command frequency f, and thus a command pulse width PW isdetermined.

[0054] For example, the reduction number of the pulse width can becalculated from the frequency fa at the point a of FIG. 6 and thecommand frequency f by using an equation ΔPW(f)=k|f−fa|. Here, the valueof k which is a constant is set so that the tilt of the frequency-speedcharacteristic of the vibration type actuator is put within analmost-constant predetermined range. In the second embodiment, anymemory element is not necessary, and its substitute can be achieved byusing the element shared with another block such as the CPU or the gatearray.

[0055] Incidentally, if the pulse width is decreased only in case off>fa, the same effect as that shown in FIG. 7 of the first embodimentcan be obtained, whereby the control becomes possible as for higherspeed.

[0056] Although the apparatus for positioning control was explained inthe second embodiment, the same effect as above can be obtained if theembodiment is applied to an apparatus only for speed control.

[0057] (Third Embodiment)

[0058]FIG. 9 shows the third embodiment of the invention. It should benoted that, in the third embodiment, the explanation of the same partsas those in the first embodiment will be omitted.

[0059] According to the third embodiment, a DC power supply 1 is acontrollable power supply which can control a voltage by a digitalsignal, a voltage and other means. A memory device 4, which stores atable for a frequency and a voltage value command DCV of the DC powersupply, out-puts the command voltage DCV to the DC poser supply 1 inaccordance with a command frequency f output from a speed control block3.

[0060] As well as the first and second embodiments, to be able to securethe gain enough for the control, the third embodiment is set so that theabsolute value of the tilt of the frequency-speed characteristic of thevibration type actuator is put within a predetermined range.

[0061] That is, the voltage of the DC power supply is decreased at theupper and lower portions of the driving frequency range (high-frequencyportion and low-frequency portion within the frequency range used fordriving). Therefore, the amplitude in the part where the voltage of theDC power supply was decreased becomes small, whereby speed decreases.The effect obtained by doing so is the same as the effect in the firstembodiment.

[0062] Further, if it is to only secure the necessary gain, the voltageof the DC power supply may be decreased at a predetermined frequency,higher than the resonance frequency of the vibration type actuator, ormore. Moreover, as well as-the second embodiment, a reduction rate ofthe voltage of the DC power supply to the frequency may be calculated bythe speed control block.

[0063] (Fourth Embodiment)

[0064]FIG. 10 shows the fourth embodiment of the invention. It should benoted that, in the fourth embodiment, the explanation of the same partsas those in the first embodiment will be omitted. In FIG. 10, numeral 15denotes an oscillator which performs oscillation at a frequencyaccording to a frequency command f output from a speed control block 3such as a known VCO (voltage-controlled oscillator). Numeral 16 denotesa power amplifier to which a gain command Ga can be set externally. Amemory device 4, which stores a table for a frequency and the gaincommand Ga of the power amplifier 16, outputs the gain command Ga of thepower amplifier 16 in accordance with the frequency command f output bythe speed control block 3. As well as the first and second embodiments,to be able to secure the gain enough for the control, the fourthembodiment is arranged so that the absolute value of the tilt of thefrequency-speed characteristic of the vibration type actuator is putwithin a predetermined range.

[0065] That is, the gain of the power amplifier 16 is decreased at theupper and lower portions of the driving frequency range. The effectobtained by doing so is the same as the effect in the first embodiment.

[0066] Further, if it is to only secure the necessary gain, the gain ofthe power amplifier 16 may be decreased at a predetermined frequency,higher than the resonance frequency of the vibration type actuator, ormore. Moreover, as well as the second embodiment, a reduction rate ofthe gain of the power amplifier 16 to the frequency may be calculated bythe speed control block.

[0067] (Fifth Embodiment)

[0068]FIG. 14 is a block diagram showing the fifth embodiment of theinvention. Here, a timer 24 generates a trigger signal Tg at a constantinterval, and an up down counter 25 performs up and down count inaccordance with the trigger signal Tg from the timer 24. Further, aposition control block 2 generates a speed command Vc as shown in FIG. 5and also generates a control state signal SM.

[0069] The operation of the up down counter 25 is determined by thevalue of the control state signal SM. As shown in FIG. 15, at the timeof acceleration (SM=1), the pulse width increases from the initial pulsewidth whenever the trigger signal Tg is input, while at the time ofdeceleration (SM=3, 4), the pulse width decreases whenever the triggersignal Tg is input. Since the frequency-speed characteristic of thevibration type actuator is as shown in FIG. 4, the driving frequencydecreases most at constant speed, and, on the other hand, the drivingfrequency increases at the time of acceleration and deceleration. Atthis time, a period that the timer generates the trigger signal Tg andamounts of increase and decrease of the pulse width are appropriatelyset so that the frequency-speed characteristic of the vibration typeactuator has the curve same as the curve shown in FIG. 7. The changes ofthe frequency and the pulse width at this time are shown in FIG. 15.Also in this case, since the frequency-speed characteristic is correctedto become nearly linear, as well as the first to fourth embodiments,controllability in each speed range becomes steady.

[0070] Here, both the cases of acceleration and deceleration wereexplained, the above operation may be performed only at the time ofdeceleration (SM=3, 4) which is most important for positioning accuracy.Further, it is also effective to perform the above operation only whenSM=3 in some deceleration to prevent the stop unanticipated at lowspeed, and to set a limit value PWmin for the pulse width as shown inFIG. 15 so that the pulse generator does not output the pulse widthbelow the limit value.

[0071] In the-above description, although the pulse-width was explainedas a parameter to correct the frequency-speed characteristic of thevibration type actuator, of course, it is also effective to directlymodify the applied voltage, the voltage of the DC power supply, the gainof the linear amplifier or the like at a constant period.

What is claimed is:
 1. A control apparatus for a vibration typeactuator, which makes driving vibration at a driving unit of a vibrationmember by applying an alternating signal to an electromechanical energyconversion element and uses at least a frequency of the alternatingsignal as a speed control parameter, said apparatus comprising: adriving circuit capable of changing a driving voltage of the alternatingsignal to be applied to said electro-mechanical energy conversionelement; and a control circuit which controls said driving circuit sothat at-least an absolute value of a tilt of a frequency-speedcharacteristic of said actuator is within a desired range in a frequencyband of predetermined range.
 2. A control apparatus for a vibration typeactuator, which makes driving vibration at a driving unit of a vibrationmember by applying an alternating signal to an electromechanical energyconversion element and uses at least a frequency of the alternatingsignal as a speed control parameter, said apparatus comprising: adriving circuit capable of changing a driving voltage of the alternatingsignal to be applied to said electromechanical energy conversionelement; and a control circuit which controls said driving circuit sothat an absolute value of a tilt of a frequency-speed characteristic ofsaid actuator is a predetermined value or more at least in a frequencyband of predetermined range.
 3. An apparatus according to claim 1,wherein said control circuit sets a change rate of the driving voltageto the frequency.
 4. An apparatus according to claim 2, wherein saidcontrol circuit sets a change rate of the driving voltage to thefrequency.
 5. An apparatus according to claim 1, wherein said drivingcircuit includes a switching circuit which performs on and offoperations in response to a driving pulse and applies a voltageaccording to the switching operation of said switching circuit to saidelectro-mechanical energy conversion element, and said control circuitchanges the width of the driving pulse according to the frequency sothat the absolute value of the tilt of the frequency-speedcharacteristic of said actuator is within the predetermined range.
 6. Anapparatus according to claim 2, wherein said driving circuit includes aswitching circuit which performs on and off operations in response to adriving pulse and applies a voltage according to the switching operationof said switching circuit to said electromechanical energy conversionelement, and said control circuit changes the width of the driving pulseaccording to the frequency so that the absolute value of the tilt of thefrequency-speed characteristic of said actuator is the predeterminedvalue or more.
 7. An apparatus according to claim 1, further comprisinga detection circuit which detects a speed and/or a position of saidvibration type actuator, wherein said control circuit changes thedriving voltage on the basis of detection information from saiddetection circuit if said actuator reaches a predetermined position or amovement amount.
 8. A control apparatus for a vibration type actuator,which makes driving vibration at a driving unit of a vibration member byapplying an alternating signal to an electro-mechanical energyconversion element and controls at least a frequency of an alternatingsignal as a speed control parameter, said apparatus comprising: adriving circuit capable of changing a driving voltage of the alternatingsignal to be applied to said electro-mechanical energy conversionelement; and a control circuit for at least performing control in afrequency range higher than a predetermined frequency so that thedriving voltage to be applied to said electromechanical energyconversion element by said driving circuit decreases as thepredetermined frequency becomes a higher frequency.
 9. An apparatusaccording to claim 8, wherein said control circuit decreases the drivingvoltage to be applied to said electromechanical energy conversionelement as the predetermined frequency becomes a higher frequency sothat an absolute value of a tilt of a frequency-speed characteristic incase of changing a frequency of said actuator by a unit amount is withina predetermined range or is a predetermined value or more.
 10. Anapparatus according to claim 8, wherein the driving voltage is changedby changing a driving pulse width in said driving circuit of applyingthe driving voltage to said electro-mechanical energy conversionelement.
 11. An apparatus according to claim 8, wherein the drivingvoltage is changed by changing a gain of an amplifier in said drivingcircuit of applying the driving voltage to said electromechanical energyconversion element.